Temperature calibration device having reconfigurable heating/cooling modules to provide wide temperature range

ABSTRACT

A temperature calibration device uses Peltier cells for heating and cooling. The Peltier cells are connected to a relay that connects the cells to each other in one configuration for heating and a different configuration for cooling. The Peltier cells also receive supply voltages having different magnitudes and polarities for heating and cooling. By changing the manner in which the Peltier cells are connected to each other and using different supply voltages for heating and cooling, the cells are able to operate closer to their specified maximum temperature differential without sacrificing the useful life of the cells.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/453,713, filed Jun. 14, 2006. This application is incorporated byreference herein in its entirety and for all purposes.

TECHNICAL FIELD

This invention relates to electrically powered devices, and, moreparticularly, to temperature calibration devices using Peltier cells toprovide heating and cooling.

BACKGROUND OF THE INVENTION

A wide variety of electrically powered heating devices are in existenceto provide a wide variety of functions. For example, temperaturecalibration devices, known as dry well calibrators, are commonly used inindustry to calibrate precision temperature probes.

Conventional dry well calibrators use thermoelectric heating/coolingmodules generally containing Peltier cells to heat or cool thecalibration probes to temperatures that can be set by a user. Electricalpower having one polarity is applied between the first and secondsubstrates of the Peltier cells to cause the temperature of the firstsubstrate to rise relative to the temperature of the second substrate,thereby heating the temperature probe being calibrated. Electrical powerhaving the opposite polarity causes the temperature of the firstsubstrate to fall relative to the temperature of the second substrate,thereby cooling the temperature probe being calibrated.

Peltier cells used in dry well calibrators are usually stacked on top ofeach other to provide heating and cooling over a range of temperaturesthat is wider than the temperature differential of each cell. The totaltemperature differential of a heating/cooling module is substantiallyequal to the sum of the temperature differentials that can be developedacross all of the stacked Peltier cells. The temperature differentialthat can be developed between the substrates of each Peltier cell islimited to a specified maximum temperature. Therefore, the limitingfactor in the operating range of a dry well calibrator is the maximumspecified temperature differential of the Peltier cells used in the drywell calibrator. This limiting effect on the operating range of dry wellcalibrators is exacerbated by the unequal heating of the Peltier cells.Specifically, the temperature differential of Peltier cells in theoutside of a stack tend to be greater than the temperature differentialof cells that are located toward the inside of the stack. To limit thetemperature differential of the cells at the outside of the stack to thespecified maximum temperature differential, the other cells in the stackare usually well below the maximum specified temperature differential.Therefore, the maximum operating range of dry well calibrators istypically much smaller than the maximum range that would be possible ifall of the Peltier cells in a stack had the same temperaturedifferential.

The need for dry well calibrators to operate over wide temperatureranges frequently requires that the Peltier cells used in thecalibrators be operated at or near their maximum specified temperaturedifferential. Unfortunately, operation of the Peltier cells at or neartheir maximum specified temperature differentials can severely limit theuseful life of the cells. Frequent replacement of the Peltier cells canbe very expensive, not only because of the cost of the cells, but alsobecause of the cost of labor required to disassemble dry wellcalibrators to replace the cells and the downtime cost during suchreplacement. As a result, there is an inevitable tradeoff betweenachieving a wide operating range for dry well calibrators and achievingreliable performance.

There is therefore a need for a dry well calibrator using Peltier cellsthat can operate over a wide range of temperatures without undulylimiting the useful life of the Peltier cells.

SUMMARY OF THE INVENTION

A temperature calibration device includes a block of thermallyconductive material that is placed in thermal communication with adevice to be calibrated. The block is in thermal contact with aplurality of Peltier cells that are connected to a configurableconnection device, which may be a relay. The configurable connectiondevice connects the Peltier cells to each other in a first configurationresponsive to a first control signal, and it connects the Peltier cellsto each other in a second configuration that is different from the firstconfiguration responsive to a second control signal. A power supplyapplies to the Peltier cells a first voltage responsive to the firstcontrol signal and a second voltage responsive to the second controlsignal. The second voltage has a polarity that is different from thepolarity of the first voltage, and it may also have a magnitude that isdifferent from the magnitude of the first voltage. A control circuit isused to generate the first and second control signals. The controlcircuit generates the first control signal when the device to becalibrated is to be cooled, and it generates the second control signalwhen the device to be calibrated is to be heated. Therefore, the Peltiercells are connected to each other in different configurations forheating and cooling, and the cells may receive voltages having differentmagnitudes for heating and cooling.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded isometric view of some of the internal componentsof a temperature calibration device according to one example of theinvention.

FIG. 2 is a cross-sectional view of the internal components of thetemperature calibration device shown in FIG. 1.

FIG. 3 is an exploded isometric view of a case surrounding the internalcomponents of the temperature calibration device shown in FIG. 1.

FIG. 4 is a front elevational view of the temperature calibration deviceof FIG. 1.

FIG. 5 is a block diagram of a system for driving Peltier cells in thetemperature calibration device of FIGS. 1-4 according to one example ofthe invention.

FIG. 6 is a block diagram showing the manner in which the system of FIG.5 connects the Peltier cells to each other when the temperaturecalibration device is to be used to cool a device to be calibrated.

FIG. 7 is a block diagram showing the manner in which the system of FIG.5 connects the Peltier cells to each other when the temperaturecalibration device is to be used to heat a device to be calibrated.

FIG. 8 is a block diagram showing the manner in which Peltier cells areconnected to each in a prior art temperature calibration device.

DETAILED DESCRIPTION

Embodiments of the present invention are directed to a system and methodfor allowing a dry well calibrator to operate over a wide range oftemperatures without adversely affecting the service life of Peltiercells used in the calibration device. Certain details are set forthbelow to provide a sufficient understanding of the invention. However,it will be clear to one skilled in the art that the invention may bepracticed without these particular details. In other instances,well-known circuits, control signals, and timing protocols have not beenshown in detail in order to avoid unnecessarily obscuring the invention.

The internal components of a heating block assembly for a typical drywell calibrator 10 are shown in FIG. 1. The dry well calibrator 10includes a cylindrical insert 14 having one or more cylindrical bores 16a,b,c sized to receive temperature probes “P” having correspondingdimensions. The insert 14 is typically manufactured from a thermallyconductive metal. The insert 14 fits into a cylindrical bore 18 formedin a heated/cooled block 20 of a suitable material, such as a metal withgood thermal conduction properties. The block 20 has a configurationthat is rectangular in both vertical and horizontal cross-section. Theinside diameter of the bore 18 is only slightly larger than the outsidediameter of the insert 14 to ensure good heat conduction from the block20 to the insert 14.

With reference also to FIG. 2, a pair of upper thermoelectricheating/cooling modules 30, 32 and a pair of lower thermoelectricheating/cooling modules 36, 38 are bonded to opposite surfaces of theblock 20. Each of the thermoelectric heating/cooling modules 30-38includes a first Peltier cell 40 having an inner substrate 42 (FIG. 1)bonded to the block 20. A second Peltier cell 44 has an inner substrate46 (FIG. 1) that is bonded to an outer substrate 48 (FIG. 1) of thefirst cell 40. Temperature conductive plates 50 are bonded to outersubstrates 54 (FIG. 1) of the second cells 44. A pair of Peltier cells60, 62 each having inner and outer substrates 66, 68, respectively,(FIG. 1) have their inner substrates 66 bonded to an outer surface ofthe plates 50. The Peltier cells 60, 62 are positioned so that theirabutting edges overlie the centers of the first and second Peltier cells40, 44. Finally, conductive leads (not shown) supply electrical power tothe Peltier cells 40, 44, 60, 62. As is well-known in the art,electrical power having one polarity causes the temperature of the innersubstrates to rise relative to the temperature of the outer substratesthereby heating the block 20. Electrical power having the oppositepolarity causes the temperature of the inner substrates to fall relativeto the temperature of the outer substrates, thereby cooling the block20. When the Peltier cells 40, 44, 60, 62 are used for either heating orcooling, the resulting temperature changes imparted to the outersurfaces 68 of the Peltier cells are moderated by heat sinks 74 abuttingthe outer substrates 68 (FIG. 1) of the cells 60, 62.

With reference also to FIG. 3, the above-described components of the drywell calibrator 10 are surrounded by an outer case 80 formed by casesections 80 a,b,c,d. The case section 80 d contains control circuitry 82that is connected to the Peltier cells 40, 44, 60, 62 for controllingthe supply of power to the cells. Two fan assembly modules 84 containinga fan 86 are positioned inside the case section 80 a so that the fan 86is behind a grill 88. The case 80 is separated from the heat sinks 74 bya space, and the fan 86 provides airflow through this space to removeheat from or supply heat to the heat sinks 74.

As best shown in FIG. 4, a keypad 90 mounted on a panel 92 of the casesection 80 a is connected to the control circuitry 82 in the casesection 80 d (FIG. 3) to control the operation of the dry wellcalibrator 10. A display 94, which is also connected to the controlcircuitry 82 in the case section 80d (FIG. 3), provides informationabout the operation of the dry well calibrator 10, such as thetemperature of the block 20.

In operation, the keypad 90 (FIG. 4) is used to set the temperature ofthe block 20 as well as the rate at which the temperature of the block20 is changed to reach the set temperature. If the temperature set bythe keypad 90 is for a temperature above ambient temperature, powerhaving a first polarity is applied to wires that are connected to thePeltier cells 40, 44, 60, 62, thereby causing the cells to cool theblock 20. If the temperature set by the keypad 90 is for a temperaturebelow ambient temperature, power having a first polarity is applied towires that are connected to the Peltier cells 40, 44, 60, 62 to causethe cells to cool the block 20. Once the temperature of the block 20 hasstabilized, the temperature probe P (FIG. 1) is inserted into acorresponding sized bore 16 of the insert 14. The probe P is thencalibrated by ensuring that a readout device (not shown) connected tothe probe P indicates the temperature of the probe P is equal to the settemperature of the dry well calibrator 10.

As explained above, the operating range of the dry well calibrator 10 islimited by the maximum specified temperature differentials of thePeltier cells 40, 44, 60, 62 and the unequal heating of the Peltiercells 40, 44, 60, 62. Balancing the temperature differentials of thePeltier cells 40, 44, 60, 62 allows the dry well calibrator 10 tooperate over a wide temperature range without the temperaturedifferential of any of the cells 40, 44, 60, 62 approaching the maximumspecified temperature differential. It has been discovered that thetemperature differentials of the Peltier cells 40, 44, 60, 62 can beequalized by driving the cells 40, 44, 60, 62 differently for coolingpurposes than they are driven for heating purposes. In particular, theexcessive temperature differential of the center Peltier cells 44compared to the temperature differential of the other cells 40, 60, 62is more of a problem when the Peltier cells 40, 44, 60, 62 are used forheating the block 20 than it is when they are used for cooling the block20.

One embodiment of a system 100 for driving the Peltier cells 40, 44, 60,62 in the upper heating/cooling modules 30, 32 in a more balanced manneris shown in FIG. 5. A second system that is identical to the system 100is used for driving the Peltier cells 40, 44, 60, 62 in the bottomheating/cooling modules 36, 38. The system 100 includes a relay driver110 that receives a control signal H/C* signal from the controlcircuitry 82 (FIG. 3). Also included in the system are a relay 120 forreconfiguring the connections between the Peltier cells 44, 60, 62, anda relay 130 for applying voltages from a power supply 140 to the Peltiercells 40, 44, 60, 62. More specifically, the relay 130 applies a coolingvoltage of +48 volts to the Peltier cells 44, 60, 62 and a coolingvoltage of +6 volts to the Peltier cells 40. The relay 130 applies aheating voltage of −24 volts to the Peltier cells 44, 60, 62 and aheating voltage of −12 volts to the Peltier cells 40. The relays 120,130 are both driven by a signal from the relay driver 110.

When the control circuitry 82 applies a low H/C* signal to the relaydriver 110 to cool the block 20, the relay 120 connects the Peltiercells 44, 60, 62 as shown in FIG. 6. In this configuration, the middlePeltier cells 44 are connected in parallel with each other, and thisparallel combination is connected in series with the Peltier cells 60,62. When the H/C* signal is low, the relay 130 applies +48 volts to thiscombination of the Peltier cells 44, 60, 62, and it applies +6 volts tothe series combination of the inner Peltier cells 40.

When the control circuitry 82 applies a high H/C* signal to the relaydriver 100 to heat the block, the relay connects the Peltier cells 44,60, 62 as shown in FIG. 7. In this configuration, the middle Peltiercells 44 are connected in series with the Peltier cells 60, 62. When theH/C* signal is high, the relay 130 applies −24 volts to this combinationof the Peltier cells 44, 60, 62, and it applies −6 volts to the seriescombination of the inner Peltier cells 40.

Assuming each of the Peltier cells 40, 44, 60, 62 have a resistance ofR, the total current drawn by the Peltier cells 44, 60, 62 when they areconfigured for cooling as shown in FIG. 6 is +48/4.5R, which is equal to+10.67/R. Therefore, the current drawn by the Peltier cells 44 is halfthat current, or +5.33/R. The total current drawn by the Peltier cells40 is +6/2R, which is equal to +3/R. In this cooling configuration, thecurrent through and the power dissipated by each of the Peltier cells40, 44, 60, 62 is as shown in Table 1, below. The current drawn by thePeltier cells 44, 60, 62 when they are configured for heating as shownin FIG. 7 is −24/6R, which is equal to −4/R. The current drawn by thePeltier cells 40 is −12/2R, which is equal to −6/R. The current throughand the power dissipated by each of the Peltier cells 40, 44, 60, 62 isalso shown in Table 1, below.

TABLE 1 Heating Cell Current Cooling Current Heating Power Cooling Power40 −6/R    +3/R 36/R    9/R 44 −4/R  +5.33/R 16/R  28.44/R 60, 62 −4/R+10.67/R 16/R 113.77/R

As can be seen from Table 1, the Peltier cells 40 dissipate more powerfor heating than they do for cooling, but the Peltier cells 44 dissipateless power for heating than they do for cooling, and the Peltier cells60, 62 dissipate much less power for heating than they do for cooling.Further, for cooling, the power dissipated by the Peltier cells 40, 44,60, 62 increases from the inner Peltier cell 40 to the outer Peltiercells 60, 62, but, for heating, decreases from the inner Peltier cell 40to the outer Peltier cells 60, 62. Therefore, in both heating andcooling, the power dissipated by the Peltier cells 40, 44, 60, 62increases from the cooled surface to the heated surface.

In contrast, prior art dry well calibrators use Peltier cells that areconnected to each other is shown in FIG. 8, in which the Peltier cells40′, 44′, 60′, 62′ correspond to the Peltier cells 40, 44, 60, 62 shownin FIGS. 1-7 for both the upper thermoelectric heating/cooling modules30, 32 and the lower thermoelectric heating/cooling modules 36, 38. Inthe prior art configuration of the Peltier cells 40′, 44′, 60′, 62′, theconfiguration of the Peltier cells 40′, 44′, 60′, 62′ is the same forboth heating and cooling. Also, the ±96 volt power applied to thePeltier cells 40′, 44′, 60′, 62′ has the same magnitude for both heatingand cooling so that the current through and power dissipated by thecells is the same for both heating and cooling. The current through thePeltier cells 40′, 44′, and 60′, 62′ is 2.74/R, 5.49/R and 10.07/R,respectively. Therefore, the current through and power dissipated by thePeltier cells 40′, 44′, 60′, 62′ in one prior art dry well calibrator issimilar to the current through and power dissipated by the Peltier cells40, 44, 60, 62 for cooling but quite different from the current throughand power dissipated by the Peltier cells 40, 44, 60, 62 for heating. Byallowing the Peltier cells 40, 44, 60, 62 to be reconfigured for heatingand cooling and/or by applying voltage having different magnitudes tothe Peltier cells 40, 44, 60, 62 for heating and cooling, the operatingrange of the dry well calibrator 10 can be maximized without operatingthe Peltier cells 40, 44, 60, 62 at or beyond their maximum specifiedtemperature differential. As a result, the dry well calibrator 10 canoperate over a large range of temperatures without sacrificingreliability.

Although the present invention has been described with reference to thedisclosed embodiments, persons skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. Such modifications are well within the skillof those ordinarily skilled in the art. Accordingly, the invention isnot limited except as by the appended claims.

1. A method of operating a temperature calibration device having aplurality of Peltier cells to heat or cool a device to be calibrated,the method comprising: connecting the plurality of Peltier cells to eachother in a first configuration when the Peltier cells are to cool thedevice to be calibrated; applying a first voltage having a firstpolarity to the Peltier cells that are in the first configuration whenthe Peltier cells are to cool the device to be calibrated; connectingthe plurality of Peltier cells to each other in a second configurationwhen the Peltier cells are to heat the device to be calibrated, thesecond configuration being different from the first configuration; andapplying a second voltage having a second polarity to the Peltier cellsthat are in the second configuration when the Peltier cells are to heatthe device to be calibrated. 2-25. (canceled)